The 2015 Audi S8 has a very tough act to follow. After all, it's based on the regular A8, which is already one of the best sedans in the world. But the folks at Audi clearly thought there was room for improvement, and the S8's twin-turbo V8 offers 520 robust reasons why they were right.

We're talking about horsepower, of course, which is about the only thing that the standard A8 doesn't have in abundance. Unless you spring for a long-wheelbase A8 L with the rare W12 engine, 435 hp is the best you can do. You might think that would be more than enough, but these days, it's barely adequate in this exclusive segment. There's no doubt about the 520-hp S8's speed, though -- it's a seriously swift sedan by practically any measure.

Naturally, the engine isn't the only thing that's special about the S8. Subtle styling tweaks abound inside and out, and both the suspension and brakes have been sportified. Yet the S8 is hardly a punishing performance sedan; on the contrary, it's basically an A8 with the world-class acceleration it deserves. In other words, cars don't get much better than this.

What's New for 2015?

The 2015 S8 gets a standard head-up display and an available sport exhaust system.

Notable options include a sport exhaust system, a Bang & Olufsen audio system (with motorized dash-mounted tweeters), adaptive cruise control and a night vision assistant, which identifies pedestrians in the dark at a distance of up to 300 feet.

Alas, the S8 shares the A8's compact-sized trunk (13.2 cu ft.), so if you think that a big Audi sedan must have a boot to match, think again.

Driving Impressions

The regular A8 already had one of the nicest cabins in the automotive world, and the S8 differs only in its sporting accents. Unique carbon trim adorns the dashboard, and the S8 badges and classic gray-faced S gauges never let you forget that are you're driving something special. The materials used are uniformly superb; practically everything looks and feels expensive. The 8-in power-retracting MMI display screen is a work of art, especially when you call up Google Earth via the navigation system to get a 3D street view of your destination.

If you're familiar with Audi's other high-end S cars, the S6 and S7, you'll notice that the S8's engine has the same displacement (4.0 liters) but makes considerably more power. Audi wanted its flagship sedan to stand apart. Indeed, the S8 actually accelerates more quickly than its smaller, lighter brethren, even though it uses a conventional 8-speed automatic transmission instead of a dual-clutch automated manual. Audi projects a 0-to-60 mph sprint of just 3.9 seconds.

But speed is cheap and plentiful these days, so what really sets the S8's drivetrain apart is its refinement? There's a supreme confidence in the way the S8 gathers velocity, reminding us of a high-speed German train more than anything else. This ultra-refined power defines the car's character, making it feel massively capable in virtually any situation.

On the handling front, the S8 isn't dramatically better than the already agile A8, but its standard sport rear differential, sport-tuned dampers and meaty 21-in tires maximize grip in all situations, whether you're navigating a rainstorm or attacking corners on your favorite road. Predictably, the ride is firm atop such large, low-profile rubber, but the adjustable suspension makes it more than tolerable.

Other Cars to Consider

BMW 7 Series -- BMW doesn't have a direct S8 equivalent, but the twin-turbocharged 750i with the Sport package is pretty similar. And the exclusive Alpina B7 might do the Audi one better.

Mercedes-Benz S-Class -- The new S-Class can't keep up with the Audi on tight roads, but in a straight line, few cars can touch the big Benz (especially in AMG trim).

Porsche Panamera -- If you don't mind driving a hatchback, and specifically a rather controversially styled hatchback, make sure you give the Panamera a shot. It's arguably the most athletic luxo-liner on the road today.

Used Rolls-Royce Ghost -- The Ghost isn't nearly as nimble as the S8, but has peerless brand cachet, and you might be able to find a certified pre-owned model at an appealing price.

AutoTrader's Advice

How could we not recommend the 2015 S8? It's one of the finest automobiles in the world. If you're lucky enough to own one, you'll likely never regret it.

This image is a stock photo and is not an exact representation of any vehicle offered for sale. Advertised vehicles of this model may have styling, trim levels, colors and optional equipment that differ from the stock photo.